As media access control for wireless LAN system, access control standardized by IEEE (The Institute of Electrical and Electronics Engineers) 802.11 systems have been widely known so far. International Standard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications or the like has described the details of the IEEE802.11.
Networking in the IEEE802.11 is based on a concept of a BSS (Basic Service Set). Two kinds of BSS are available, that is, BBS defined by the infrastructure mode in which a master control station such as an access point (Access Point: AP) exists and IBSS (Independent BSS) defined by the ad hoc mode composed of only a plurality of mobile terminals (Mobile Terminal: MT).
Operations of the IEEE802.11 in the infrastructure mode will be described with reference to FIG. 30. In the BSS in the infrastructure mode, an access point for performing coordination should be absolutely provided within a wireless communication system. In FIG. 30, assuming that a communication station SAT0, for example, is a communication station SA which functions as an access point, then BSSes within a range of radio waves near the local station are collected to construct a cell in the so-called cellular system. Mobile stations (SAT1, SAT2) existing neat the access point are accommodated into the access point and joined the network as a member of the BSS. The access point transmits a control signal called a beacon at a proper time space. A mobile terminal that can receive this beacon recognizes that the access points exists near it and establishes connection between it and the access point.
The communication station SATO, which is the access point, transmits a beacon (Beacon) at a predetermined period space as shown on the right-hand side of FIG. 30. The next beacon transmission time is sent into the beacon by a parameter called a target beacon transmit time (TBTT: Target Beacon Transmit Time). When a time reaches the TBTT field, the access point activates a beacon transmission procedure. Also, since a neighboring mobile terminal receives a beacon and is able to recognize the next beacon transmission time by decoding the inside TBTT field, depending on the cases (mobile terminal need not receive information), the receiver may be de-energized until the next TBTT field or a plurality of future target beacon transmission times and the mobile terminal may be placed in the sleep mode.
This specification principally considers the gist of the present invention in which the network is operated without application of a master control station such as the access point, and hence the infrastructure mode will not be described any more.
Next, communication operations according to the IEEE802.11 in the ad hoc mode will be described with reference to FIGS. 31 and 32.
On the other hand, in the IBSS in the ad hoc mode, after each communication station (mobile terminal) has negotiated with a plurality of communication stations, each communication station defines the IBSS independently. When the IBSS is defined, the communication station group determines the TBTT at every constant interval after negotiations. When each communication station recognizes the TBTT with reference to a clock within the local station, if it recognizes that other communication station has not transmitted the beacon after a delay of a random time, then the communication station transmits the beacon. FIG. 31 shows an example of the case in which two communication stations SAT1, SAT2 constitute the IBSS. Accordingly, in this case, any one of communication stations belonging to the IBSS is able to transmit the beacon at each arrival of the TBTT field. Also, it is frequently observed that the beacons will conflict with each other.
Further, also in the IBSS, according to the necessity, each communication station is placed in the sleep mode in which a power switch of its transmission and reception unit is turned off. A signal transmission and reception procedure in this case will be described with reference to FIG. 32.
In the IEEE82.11, when the sleep mode is applied to the IBSS, a certain time period from the TBTT is defined as an ATIM (Announcement Traffic Indication Message) Window (hereinafter referred to as an “ATIM window”).
During the time period of the ATIM window, since all communication stations belonging to the IBSS are operating the reception units, even the communication station which is being operated in the sleep mode fundamentally is able to receive communication in this time period. When each communication station has its own information for other communication station, after a beacon has been transmitted in the time period of this ATIM window, the communication station lets the reception side know that the communication station has its own information for other communication station by transmitting the ATIM packet to other communication station. The communication station, which has received the ATIM packet, causes the reception unit to continue operating until the reception from the station that has transmitted the ATIM packet is ended.
FIG. 32 shows the case in which three communication stations STA1, STA2, STA3 exist within the IBSS, by way of example. As shown in FIG. 32, at the time TBTT, the respective communication stations STA1, STA2, STA3 operate back-off timers while monitoring the media state over a random time. The example of FIG. 32 shows the case in which the communication station STA1 transmits the beacon after the timer of the communication station STA1 has ended counting in the earliest stage. Since the communication station STA1 transmits the beacon, other two communication stations STA2 and STA3 do not transmit the beacon.
The example of FIG. 32 shows the case in which the communication station STA1 holds information for the communication station STA2, the communication station STA2 holding information for the communication station STA3. At that time, as shown in FIGS. 32B, 32C, after having transmitted/received the beacons, the communication stations STA1 and STA2 energize the back-off timers while monitoring the states of the media again over the random time, respectively. In the example of FIG. 32, since the timer of the communication station STA2 has ended counting earlier, first, the communication station STA2 transmits the ATIM message to the communication station STA3. As shown in FIG. 32A, when receiving the ATIM message, the communication station STA3 feeds the message of the reception back to the communication station STA2 by transmitting an ACK (Acknowledge) packet which is an acknowledge packet to the above communication station. After the communication station STA3 has finished transmitting the ACK packet, the communication station STA1 further energizes the back-off timer while monitoring the respective states of the media over the random time. When the timer finishes counting after a time set by the timer has passed, the communication station STA1 transmits the ATIM packet to the communication station STA2. The communication station STA2 feeds the message of the reception back to the communication station STA1 by returning the ACK packet to the above communication station.
When the ATIM packet and the ACK packet are exchanged within the ATIM window, also during the following interval, the communication station STA3 energizes the receiver to receive information from the communication station STA2, and the communication station STA2 energizes the receiver to receive information from the communication station STA1.
When the ATIM window is ended, the communication stations STA1 and STA2 which hold the transmission information energize the back-off timers while monitoring the respective states of the media over the random time. In the example of FIG. 32, since the timer of the communication station STA2 has finished counting first, the communication station STA2 first transmits the information to the communication station STA3. After this transmission of the information was ended, the communication station STA1 energizes the back-off timer while monitoring again the respective states of the media over the random time, and after the timer is ended, it transmits the packet to the communication station STA2.
In the above-mentioned procedure, a communication station which has not received the ATIM packet within the ATIM window or which does not hold information de-energizes the transmitter and receiver until the next TBTT field and it becomes possible to decrease power consumption.
Next, the access contention method of the IEEE802.11 system will be described with reference to FIG. 33. In the above explanation, while we have described “communication station energizes the back-off timer while monitoring the states of the media over the random time”, let us make additional explanation to this case.
In the IEEE802.11 system, four kinds of IFS are defined as packet spaces (IFS: Inter Frame Space) extending from the end of the immediately-preceding packet to the transmission of the next packet. Of the four kinds of the inter frame spaces, three inter frame spaces will be described. As shown in FIG. 33, as the IFS, there are defined SIFS (Short IFS), PIFS (PCF IFS) and DIFS (DCF IFS) in the sequential order of short inter frame space. According to the IEEE802.11, a CSMA (Carrier Sense Multiple Access) is applied as the fundamental media access procedure. Accordingly, before the transmission unit transmits some information, the communication station energizes the backoff timer over the random time while monitoring the state of the media. If it is determined that the transmission signal does not exist during this time period, then the transmission unit is given a transmission right.
When the communication station transmits the ordinary packet in accordance with the CSMA procedure (called a DCF: Distributed Coordination Function), after the transmission of some packet has been ended, the state of the media of only the DIFS is monitored. Unless the transmission signal exists during this time period, then the random backoff is made. Further, unless the transmission signal exists during this time period, the transmission unit is given a transmission right. On the other hand, when a packet such as ACK packet which has an exceptionally large emergency is transmitted, the transmission unit is allowed to transmit the packet after the SIFS packet space. Thus, it becomes possible to transmit the packet with the large emergency before the packet that is to be transmitted in accordance with the ordinary CSMA procedure. Different kinds of packet spaces IFS are defined for this reason. Packet transmission contention is prioritized depending upon whether the IFS is the SIFS or the PIFS or the DIFS. The purpose of using the PIFS will be described later on.
Next, the RTS/CTS procedure in the IEEE802.11 will be described with reference to FIGS. 34 and 35. In network under the ad hoc environment, it is generally known that a problem of a hidden terminal arises. As a methodology for solving the most part of this problem, there is known a CSMA/CA based upon the RTS/CTS procedure. The IEEE802.11 also uses this methodology.
An example of operation in the RTS/CTS procedure will be described with reference to FIG. 34. FIG. 34 shows an example of the case in which some information (DATA) is transmitted from a communication station STA0 to a communication station STA1. Before transmitting actual information, the communication station STA0 transmits an RTS (Request To Send) packet to the communication station STA1 which is an information destination station in accordance with the CSMA procedure. When the communication station STA1 received this packet, it transmits a CTS (Clear To Send) packet which feeds information indicative of the reception of the RTS packet back to the communication station STA0 to the communication station. When the communication station STA0 which is the transmission side receives the CTS packet without accident, the communication station regards that the media is clear and transmits an information (Data) packet immediately. After the communication station STA1 receives this information packet without accident, it returns the ACK packet and the transmission of one packet is ended.
Actions that will occur in this procedure will be described with reference to FIG. 35. In FIG. 35, it is assumed that a communication station STA2 may transmit information to a communication station STA3. Having confirmed by the CSMA procedure that the media is clear during a predetermined period, the communication station STA2 transmits the RTS packet to the communication station STA3. This packet is also received by the neighbor communication station STA1 of the communication station STA2. Because the communication station STA1 receives the RTS packet and becomes aware that the station STA2 intends to transmit some information, it recognizes that the media is occupied by the station STA2 until the transmission of such information is ended, and it also becomes aware of the fact that the media is occupied without monitoring the media during this time period. This work is called an NAV (Network Allocation Vector). The RTS packet and the CTS packet have durations of time in which the media is occupied in the transaction written thereon.
Returning to the description, having received the RTS packet transmitted from the communication station STA2 to the communication station STA3, the communication station STA1 becomes aware of the fact that the media is placed in the occupied state during a time period designated by the RTS packet, and hence it refrains from transmitting information. On the other hand, the communication station STA3 which received the RTS packet returns the CTS packet to the communication station to feed information indicative of the reception of the RTS packet back to the communication station STA2. This CTS packet is also received by a neighbor communication station STA4 of the communication station STA3. The communication station STA4 recognizes by decoding the content of the CTS packet that information is transmitted from the communication station STA2 to the communication station STA3, and it becomes aware of the fact that the media will be occupied during a time period designated by the CTS packet. Hence, it refrains from transmitting information.
When the above-described RTS packet and CTS packet are transmitted and received, the transmission is prohibited between “neighboring station of the communication station STA2 which is the transmission station” which could receive the RTS packet and “neighboring station of the communication station STA3 which is the reception station” which could receive the CTS packet, whereby information can be transmitted from the communication station STA2 to the communication station STA3 and the ACK packet can be returned without being disturbed by the sudden transmission from the neighboring station.
Next, a band reserve means in the IEEE802.11 system will be described with reference to FIG. 36. In the above-mentioned IEEE802.11 system access control, access contention based on the CSMA procedure is executed, and hence it is impossible to guarantee and maintain a constant band. In the IEEE802.11 system, a PCF (Point Coordination Function) exists as a mechanism for guaranteeing and maintaining the band. However, the basis of the PCF is polling and it does not operate in the ad hoc mode but it operates only in the infrastructure mode under control of the access point. Specifically, in order to execute the access control while the band is being guaranteed, a coordinator such as an access point is required and all controls are carried out by the access point.
For reference, operations of the PCF will be described with reference to FIG. 36. In FIG. 36, it is assumed that the communication station STA0 is the access point and that the communication stations STA1 and STA2 joined in the BSS managed by the access point STA0. Also, it is assumed that the communication station STA1 transmits information while it guarantees the band.
Having transmitted the beacon, for example, the communication station STA0 performs polling to the communication station STA1 at the SIFS space (CF-Poll in FIG. 36). The communication station STA1 which received the CF-Poll is given a right to transmit data and is thereby allowed to transmit data at the SIFS space. As a result, the communication station STA1 transmits the data after the SIFS space. When the communication station STA0 returns the ACK packet for the transmitted data and one transaction is ended, the communication station STA0 again performs polling to the communication station STA1.
FIG. 36 shows also the case in which polling of this time is failed due to some reason, that is, the state in which the polling packet shown as the CF-Poll follows the SIFS space. Specifically, when the communication station STA0 becomes aware that no information is transmitted from the communication station STA1 after the SIFS space elapsed since it has performed polling, it regards that the polling is failed and performs polling again after the PIFS space. If this polling is successful, then data is transmitted from the communication station STA1 and the ACK packet is returned. Even when the communication station STA2 holds the transmitted packet during a series of this procedure, since the communication station STA0 or STA1 transmits information at the SIFS or PIFS space before the DIFS time space elapses, the right to transmit information is never moved to the communication station STA2 and hence the communication station STA1 to which the polling is performed is constantly given a priority.
Official Gazette of Japanese laid-open patent application No. 8-98255 discloses an example of access control of such wireless communication.
When access control of wireless communication is carried out without such master control station (access point), as compared with the case in which communication is carried out with the master control station, there were various restrictions. To be concrete, the following problems arise.
Problem 1: Selection of Coordinator
For example, as shown in FIG. 37, let it be assumed that a network is configured by the above-mentioned IEEE802.11 system when communication stations 10 to 17 are located in the scattered state and communication ranges 10a to 17a in which the communication stations 10 to 17 can directly communicate with each other. In such case, if the network is configured in the infrastructure mode, then there arises a problem of how to select a communication station that should be operated as the access point (coordinator). In the IEEE802.11 system, a communication station accommodated within the BSS may communicate with only a communication station which belongs to the same BSS, and the access point is operated as a gateway to other BSS. In order to efficiently make networking on the whole of the system, there are various arguments such as to select which location of the communication station as the access point or how to configure again the network when the access point is de-energized. Although it is desirable that the network could be configured without the coordinator, the infrastructure mode of the IEEE802.11 system cannot meet with such requirements.
Problem 2: Disagreement of Achievable Area
In the ad hoc mode of the IEEE802.11 system, although the network can be configured without the coordinator, it is assumed that the IBSS is constructed by a plurality of communication stations located at the surrounding areas. For example, as shown in FIG. 37, it is assumed that the communication stations 10, 11, 12, 13 (STA0, STA1, STA2, STA3) are accommodated within the same IBSS. Then, although the communication station 11(STA1) can communicate with the communication stations 10, 12, 13 (STA0, STA2, STA3), the communication station 10 (STA0) cannot directly communicate with the communication station 12 (STA2). In such case, according to the beacon transmission procedure of the IEEE802.11 system, it is frequently observed that the communication station 10 (STA0) and the communication station 12 (STA2) transmit the beacons at the same time, and at that time, the communication station 11 (STA1) becomes unable to receive a beacon, which causes a problem.
Further, as shown in FIG. 37, for example, let it be assumed that the communication stations 15, 16, 17 (STA5, STA6, STA7) constitute an IBSS (IBSS-A) and that the communication stations 10, 11, 12, 13 (STA0, STA1, STA3, STA3) constitute an IBSS (IBSS-B). At that time, since the two IBSSes are operating completely independently, an interference problem does not arise between the two IBSSes. Here, let it be considered the case in which a new communication station 14 (STA4) appears on the network. Then, the communication station 14 (STA4) is able to receive both signals from the IBSS-A and the IBSS-B. When the two IBSSes are coupled together, although the communication station STA4 can enter both of the IBSS-A and the IBSS-B, the IBSS-A is operated in accordance with the rule of the IBSS-A and the IBSS-B is operated in accordance with the rule of the IBSS-B. Then, there is a possibility that collision of the beacons and collision of the ATIM packets will occur, which also raises a problem.
Problem 3: Method of Realizing Power Save Mode
In the ad hoc mode, the power save mode can be realized by transmitting the ATIM packets with each other within the ATIM window according to the random access. When information to be transmitted is a small amount of information such as bits, an overhead required by the ATIM packets increases, and a methodology in which the ATIM packets are to be exchanged according to the random access is very inefficient.
Problem 4: Band Reserve in Network without Coordinator
Also, according to the IEEE802.11 system, in the ad hoc mode, a mechanism for carrying out band reserve does not exist, and hence there is no method but to constantly follow the operation of the CSMA procedure.
Problem 5: Incompleteness of RTS/CTS Procedure
In the RTS/CTS procedure of the IEEE802.11 system, not only a communication station which received the CTS packet but also a communication station which received the RTS packet is prohibited from transmitting information. However, in the case shown in FIG. 35, the station that is prohibited from transmitting information is only the communication station STA4 and the communication station STA1 does not affect “transmission of DATA from the communication station STA2 to the communication station STA3”. In the RTS/CTS procedure, to prohibit the communication station which received the RTS packet from transmitting information requires a large margin to the safety side and this is one of the factors which degrade a system throughput.
Problem 6: Considerations on Separation of BBSes by TDMA
In the scenario described in the above-mentioned Problem 2 (in FIG. 37, the communication stations STA5, STA6, STA7 constitute the IBSS (IBSS-A) and the communication stations STA0, STA1, STA2, STA3 constitute the IBSS (IBSS-B)), as a method for solving the problem which arises when the communication station STA4 appears to couple both of the IBSSes, there exists a method for separating the IBSS-A and the IBSS-B by a TDMA (Time Division Multiple Access: time division multiple access) system. An example of this case is shown in FIG. 38. This is a method used in an ARIB STD-T70 (HiSWANa) system and the like. A time zone that is exclusively used for a sub-network is constructed in a frame of some BBS. However, according to this method, spatial recycling of resources is aborted and hence utilization ratio is decreased considerably, which also causes a problem.
In view of the aforesaid aspects, it is an object of the present invention to provide excellent wireless communication system, wireless communication apparatus and wireless communication method and computer program in which the problems arising when a wireless system such as a wireless LAN is constructed as a decentralized distributed type network without control and controlled relationship such as a master station and slave stations can be solved.
Other object of the present invention is to provide excellent wireless communication system, wireless communication apparatus and wireless communication method and computer program in which data can be transmitted while collisions are being avoided in a decentralized distributed type network.
A further object of the present invention is to provide excellent wireless communication system, wireless communication apparatus and wireless communication method and computer program in which collisions of beacons can be suitably avoided among a plurality of communication stations in a network configured when communication stations transmit beacons with each other.
Yet a further object of the present invention is to provide excellent wireless communication system, wireless communication apparatus and wireless communication method and computer program in which a decentralized distributed type wireless network can be suitably formed while collisions of beacons that communication stations transmitted with each other can be avoided.